Microlens sheets are widely used in front and/or rear projection screens for television, computer monitor and/or other viewing systems, and in many other applications. As used herein, a microlens is a lens having a size which is on the order of microns, for example on the order of about 5 μm to about 1000 μm in size.
A microlens sheet that may be used for front and/or rear projection viewing systems may include a substrate having first and second opposing faces, an array of microlenses on the first face and a corresponding array of apertures on the second face. The microlenses can provide divergence of light falling on the screen along first and second directions, such as horizontal and vertical directions, to thereby fill a desired viewing space. These microlenses may generally have an anamorphic (i.e., non-rotationally symmetric) shape that may result in a broader divergence in the horizontal direction than in the vertical direction. The aperture(s) opposing each microlens may permit light which is projected on the screen to pass to the viewing space, while blocking or reducing ambient light reflecting from the screen, which can improve contrast of the projected image.
Conventional techniques for creating microlens arrays with aperture masks may involve fabrication of the arrays on suitable substrates which are or can be coated with appropriate radiation absorbing mask materials. High intensity radiation is then directed through the lenses and focused by them. If the structure of the lens array, substrate and mask has been designed so that the focal points of the lens array are at or near the mask layer, the radiation will form apertures in the mask at these focal points. See, for example, U.S. Pat. No. 4,172,219 to Deml et al., entitled Daylight Projection Screen and Method and Apparatus for Making the Same. Moreover, a non-ablative process for fabricating a light absorbing layer is described in Published United States Patent Application No. US 2004/0017612 A1, entitled Micro-Lens Array With Precisely Aligned Aperture Mask and Methods Of Producing Same, published Jan. 29, 2004, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
The microlenses may be any shape, size, or configuration provided that they cause light rays arriving from a prescribed direction to converge to a convergence zone beyond the lens. The size of the lenses may be small enough such that individual lenses generally are smaller than the size of individual image pixels projected from the image projector. The microlens shape may be constant and/or may vary across the surface of the screen, and may be lenticular, spherical, aspherical, anamorphic, prism-shaped, pyramidal shape, combinations and subcombinations thereof and/or other shapes. Microlenses may be fabricated, for example, as described in Published United States Patent Application Nos. US 2005/0058947 A1 to Rinehart et al., entitled Systems And Methods for Fabricating Optical Microstructures Using a Cylindrical Platform and a Rastered Radiation Beam; US 2005/0058948 A1 to Freese et al., entitled Systems and Methods for Mastering Microstructures Through a Substrate Using Negative Photoresist and Microstructure Masters So Produced; and/or US 2005/0058949 A1 to Wood et al., entitled Systems and Methods for Fabricating Microstructures by Imaging a Radiation Sensitive Layer Sandwiched Between Outer Layers, and Microstructures Fabricated Thereby, all published Mar. 17, 2005, and assigned to the assignee of the present invention, the disclosures of which are hereby incorporated herein by reference in their entirety as if set forth fully herein.
In a projection screen that uses a microlens sheet, a respective microlens may be designed to efficiently collect incident light, and to focus this light to a convergence point, in order to pass through the aperture and provide divergence of imaging light beyond the convergence point in a manner appropriate for the desired horizontal and vertical viewing angles of the screen. Many approaches have been proposed for the design of such microlenses, including various mathematical formulas describing the shape of the lens. In general, such lenses may have a planoconvex shape, and may have different radii of curvature depending on the specific cross section of lens that is desired. For example, a lens with an anamorphic shape may be desirable for creating highly divergent light in the horizontal light in the horizontal plane, while limiting divergence in the vertical plane. Such an anamorphic lens may have a smaller base in the horizontal plane and a larger base in the vertical plane. A design of a microlens sheet having an array of anamorphic lenses is described in Published United States Patent Application US 2004/0004770 A1 to Ebina et al., entitled Microlens Sheet and Projection Screen. As stated in the abstract of the Ebina et al. published application, in order to provide a microlens sheet in which unit lenses are disposed in highly accurate pitch so as to control the light diffusing characteristics of the light emission direction not only in a horizontal direction but also over 360 degrees by using the lens function of one piece of lens sheet, a microlens sheet has a microlens array section in which unit lenses are disposed in approximate matrix in a second dimensional manner on at least one surface of a base board, and the microlens array section is formed on only one surface of the base board, the microlens array section includes the unit lens having an aspherical shape, and disposition pitch of neighboring unit lenses is 200 μm or shorter.
For commercially desirable viewing screens, it may be desirable to produce a relatively large full-width at half maximum (FWHM) angle. In other words, it may be desirable for the light intensity to fall off with angle from normal to the substrate (0°), such that the light intensity reaches half its on axis intensity at +/− a relatively large a viewing angle from the normal. It may also be desirable for the light intensity to have a smooth distribution as viewing angle is increased, with the intensity falling monotonically (i.e., consistently decreasing without increasing), with an increase in viewing angle. Unfortunately, it may be difficult to design microlens sheets with a desired FWHM and desired intensity distribution characteristics using conventional techniques.